Method of surface-conditioning heat-treating-furnace hearth rolls having sleeves of rebonded fused silica thereon by processing silicon steel strip

ABSTRACT

Generally, the method of the invention involves surfaceconditioning new and used rebonded fused silica sleeved hearth rolls of a continuous silicon steel strip heat-treating furnace while the furnace is in production operation. Method consists of scheduling the furnace, when new rolls are to be conditioned, to process silicon steel strip product requiring dry annealing in an atmosphere having a maximum dew point of 30*F at a temperature the range of approximately 1,400* to 2,000*F (preferably 1,500*F) and processing such silicon steel strip through the furnace with the strip applying a sustained force normal to the peripheral surfaces of the rolls for a period of 24 to 72 hours. In a modified embodiment of the invention, used rolls are surfaceconditioned to remove oxide roll-pickup by scheduling the processing of silicon strip product having rough surfaces through the furnace which requires a dry anneal in an atmosphere having a maximum dew point of 30*F at a temperature ranging from 1,400* to 2,000*F (preferably 1,500*F) and processing such strip through the furnace for approximately 16 hours.

United States Patent [191 Hamilton et al.

[ Feb..5,1i974 4] METHOD OF SURFACE-CONDITIONING HEAT-TREATlNG-FURNACE HEARTH ROLLS HAVING SLEEVES 0F REBONDED FUSED SlLlCA THEREON BY PROCESSING SILICON STEEL STRIP [75] Inventors: Jack L. Hamilton, Monroeville Borough; John W. ,lenks, Franklin Twp.; John F. Milberger, Washington Twp., all of Pa.

[73] Assignee: United States Steel Corporation, Pittsburgh, Pa.

[22] Filed: Oct. 20, 1972 [21] Appl. No.: 299,533

[52] US. Cl 72/236, 29/148.4 D, 51/290,

148/ll.5 R, 148/113, 148/122 [51] Int. Cl B211) 27/04 [58] Field of Search.. 148/112, 113, 121, 122, 11.5; 266/24, 27, 30; 15/1; 29/l48.4 D; 72/236;

Primary Examiner-Hyland Bizot Assistant ExaminerW. R. Satterfield Attorney, Agent, or Firm-Donald S. Ferito 5 7] ABSTRACT Generally, the method of the invention involves surface-conditioning new and used rebonded fused silica sleeved hearth rolls of a continuous silicon steel strip heat-treating furnace while the furnace is in production Operation. Method consists of scheduling the furnace, when new rolls are to be conditioned, to process silicon steel strip product requiring dry annealing in an atmosphere having a maximum dew point of 30F at a temperature the range of approximately 1,400 to 2,000F (preferably 1,500F) and processing such silicon steel strip through the furnace with the strip applying a Sustained force normal to the peripheral surfaces of the rolls for a period of 24 to 72 hours. In a modified embodiment of the invention, used rolls are surface-conditioned to remove oxide roll-pickup by scheduling the processing of silicon strip product having rough surfaces through the furnace which requires a dry anneal in an atmosphere having a maximum dew point of 30F at a temperature ranging from 1,400 to 2,000F (preferably 1,500F) and processing such Strip through the furnace for approximately 16 hours.

7 Claims, No Drawings METHOD OF SURFACE-CONDITIONING HEAT-TREATING-FURNACE HEARTH ROLLS HAVING SLEEVES OF REBONDED FUSED SILICA THEREON BY PROCESSING SILICON STEEL STRIP The present invention relates generally to the heat treating of steel strip and more particularly to a method of surface-conditioning rolls having sleeves thereon of rebonded fused silica especially suitable for use in continuous silicon steel strip heat-treating furnaces.

Steel strip can be annealed in either coil form, commonly referred to as batch annealing or by passing the uncoiled strip through a continuous annealing furnace. The present invention addresses itself to the continuous annealing of steel strip. Continuous annealing furnaces are of two types, namely, vertical or horizontal. Vertical or horizontal refers to the strip alignment in the furnace during the heat treatment operation. The major affect on product quality in comparing the two types is the capability of the horizontal furnace to produce a stress-free strip product.

A typical horizontal continuous heat-treating furnace roller hearth line consists of an entry section including coil-payoff reels, strip welder, horizontal looping equipment, electrolytic cleaning, scrubbing and drying units. The entry section is joined to a contiguous horizontal heat-treating section consisting of a gas-fired heating zone (72 feet long), and an electrically-heated holding zone (600 feet long), a controlled cooling zone (200 feet long), and a jet-cooling zone (90 feet long). The dimensions cited pertain to a typical unit now in operation. The dimensions are stated to illustrate 'the comparative lengths of the various elements of a typical horizontal continuous annealing furnace. These dimensions may vary as desired or necessary. The exit section of the furnace consists of a horizontal looping unit and tension reel. The strip is supported as it passes through the heat-treating, section of the furnace in catenary fashion by individually-motor-driven alloy steel rolls having sleeves of rebonded fused silica thereon. This type of roll is described and claimed in pending US patent application Ser. No. 172,947, now US. Pat. No. 3,75 1,195, which is assigned to the assignee of the present application.

During the annealing of silicon steel strip in a horizontal furnace such as described above, oxides of silicon and carbon are given off as gases (C0, C while the silicon oxide (SiO is retained on the strip surface.

This silicon oxide is very much like a thin glass coating which is in a viscous state. Small portions of this oxide are sometimes picked up from the steel strip by the roll sleeve surfaces at various points on the periphery thereof. This spotty buildup on the roll sleeve surfaces causes a depression or dimple to form on strip subsequently passing over the roll surface. Such depressions or dimples are particularly objectionable on silicon steel strip since a major end use of this product is the forming of stator and rotor laminations of electrical equipment or devices. A specific number of laminations is required, for example, for building up the armature of an electric motor in order to attain certain desired electrical characteristics. Specifically, a fixed number of laminationsis required for each lineal inch of cross section of the armature in order to attain certain electrical characteristics. Therefore, distortions of the surface of silicon steel strip used for making these laminations cannot be tolerated.

The susceptibility of silicon steel strip to surface deformation or marking is related to its silicon content. Low silicon steel strip (one-half to 1% silicon content), which is relatively soft, shows the greatest susceptibility to surface marking and deformation; medium silicon steel strip (1 to 3% silicon content) has a lesser, but still objectionable, susceptibility to surface marking and deformation; and high silicon steel strip (above 3% silicon content) has the greatest resistance to surface marking or deformation by roll-pickup accretions on annealing furnace rolls.

The amount of roll-pickup and subsequent strip marking has been found to be related to annealing temperature and moisture content of the atmosphere in the continuous annealing furnace. A normal operating cycle to anneal silicon strip in a typical horizontal continuous annealing furnace would be in a range of 1 ,400 to 2,000lF with steam being introduced in the electrically-heated holding zone to decarburize the strip. A normal decarburization atmosphere would have a minimum F dew point. These dual conditions of high temperature and high dew point present the most severe pickup problem. However, the temperature of l,400 to 2,000F with a dry anneal (maximum 30F dew point) does not cause a major pickup problem. Further, annealing temperatures below 1,400F whether wet or dry cause minimal pickup problems.

Prior to our invention, it was usual practice to remove this oxide pickup from the hearth rolls by utilizing a coil of stainless steel strip which had been temperrolled to produce a rough surface on the stainless steel strip. This practice comprised unwinding the coil of stainless steel strip and feeding the strip into the furnace after which the travel of the stainless steel strip was stopped. The furnace hearth rolls were then rotated in alternate directions to grind-of the oxide pickup on the peripheral surfaces of the hearth roll sleeves by contact between the peripheral surfaces of the alternately rotated hearth rolls and the rough surface of the stationary stainless steel strip. Although this practice was usually successful in removing oxide pickup from the peripheral surfaces of the hearth roll sleeves, it obviously had a detrimental effect on the productivity of the heat-treating furnace.

Accordingly, it is the primary object of our invention to provide an improved method for surfaceconditioning the hearth rolls of a heat-treating furnace,

which rolls have sleeves of rebonded fused silica thereon, without affecting productivity of the furnace.

It is a more specific object of our invention to provide an improved method surface-conditioning new hearth rolls of a silicon strip heat-treating furnace, which rolls have sleeves of rebonded fused silica thereon, after they have been installed in a silicon strip heat-treating furnace and the furnace is in operation heat-treating silicon strip by scheduling the heat treatment of silicon strip through the furnace at start-up and for a period of 24 to 72 hours thereafter which treatment requires a dry anneal (30F maximum dew point) and a temperature of 1,400 to 2,000F (preferably 1,500F).

It is a further specificobject of our invention to provide an improved method for surface-conditioning the hearth rolls of a silicon .strip heat furnace having sleeves of fused rebonded silica thereon to remove oxide pickup from the peripheral surfaces of the roll sleeves by heat treating high silicon (approximately 3% silicon content) strip having an abrasive surface in a dry atmosphere (30F maximum dew point) at a temperature between l,400 and 2,000F (Preferably l,500F) through the furnace.

The above objects will become more apparent after referring to the following specification.

We have determined the oxide pickup on the sleeves of silicon strip heat-treating furnace hearth rolls having sleeves of rebonded fused silica thereon can be minimized by properly treating the rolls before subjecting them to the wide variety of operating conditions prevalent during normal operation of the furnace heat treating various grades of silicon strip. We have observed that newly manufactured or newly ground rebonded fused silica sleeved hearth rolls installed in the furnace and subjected to a high temperature wet anneal develop a significant amount of objectionable oxide pickup within a relatively short period of approximately three to seven days after installation.

Such a fast oxide pickup occurs because of the surface condition of the sleeves of the newly manufactured or newly ground rolls. The roll sleeves not only have grinding marks resulting from the pre-service conditioning of the sleeve surface, but the nature of the rebonded fused silica surface is such that it is also somewhat porous. Therefore, if the fresh roll is initially exposed to conditions of high temperature and high dew point, the glutinous oxide produced on the silicon strip being heat treated is picked up in the sleeve pores and the imperfections resulting from pre-service surfaceconditioning. Although the amounts of oxide pickup are minute, the oxide penetrates the porous surfaces of the sleeves and then serve as a base for further cumulative pickup. The sleeved roll must, therefore, be initially conditioned to form a compact peripheral surface which will not readily permit oxide adherence.

Use of our method for surface-conditioning newly installed fresh rolls consists of starting up operation of the furnace by initially heat-treating silicon strip therethrough which requires a dry anneal (30F maximum dew point) with a temperature between l,400 and 2,000F but preferably approximately l,500F. The processing of silicon strip requiring such a dry anneal should be continued for at least 24 hours. Additional benefits can be obtained by continuing the production of silicon strip requiring such a dry anneal up to 72 hours before starting to produce silicon strip requiring a higher temperature and a wet atmosphere in its treatment.

It would ordinarily be advantageous to increase tension in the silicon strip during continuous annealing to minimize contact between the strip surface and the peripheral surfaces of the hearth rolls. However, with the long furnace length of typical horizontal continuous silicon strip annealing furnaces, the tension which would be required to significantly reduce strip-sag would impair the desired magnetic or electrical properties being sought to be created in the strip. Such high tension would also tend to neck" the strip. The method of our invention utilizes the normal catenary disposition of the strip between hearth rolls to an advantage. The weight of the strip while operating in the dry annealing atmosphere applies a sustained force normal to the peripheral surface of the roll which. results in a polished peripheral surface on the sleeved roll which is essentially free of pores which, if present, would later attract strip oxides. Where fresh rolls are thus conditioned in accordance with the method of our invention, the annealing line will operate free of oxide pickup for at least fourteen days.

When oxide pickup does begin to develop on the sleeved hearth rolls, a modified embodiment of the method of our invention is used to remove the oxide pickup. This modified embodiment of the invention includes two alternatives depending upon the severity of oxide pickup on the rolls.

The first alternative comprises scheduling the annealing furnace to anneal high silicon (3% or more silicon content, by weight) strip which has'been previously box annealed at high temperature. In processing silicon strip which requires box annealing prior to continuous annealing, a coating of magnesium oxide is applied to the strip prior to box annealing to prevent spot welding of concentric wraps of the strip coil at the high temperature used during box annealing. During the box annealing operation, the magnesium in this coating and the silicon in the strip combine to form a magnesium silicate on the strip surface. This magnesium silicate imparts an abrasive surface to the strip. Processing this type of silicon strip through the annealing furnace for approximately sixteen hours at a low temperature of approximately 1,500F and in a dry atmosphere (30F maximum dew point) will effectively remove oxide pickup from the hearth rolls with no detrimental effect on productivity.

Alternatively, if desired, instead of processing previously box annealed high silicon strip through the continuous annealing furnace to remove oxide pickup from the rolls, the furnace could be scheduled to process high silicon strip which has been given a rough temper mill finish during its previous processing.

It should now be obvious that the loss of prime production from a silicon strip annealing furnace occasioned by removal of oxide pickup from the rebonded fused silica hearth rolls through heretofore used methods has been eliminated by the method of our invention. By surface-conditioning new sleeved hearth rolls in accordance with the method of our invention, prime product productivity is not affected. Then by manipulation of the furnace production schedule in accordance with the modified method of our invention, oxide pickup on the rolls can be eliminated as it begins to occur without interrupting operation and productivity of the furnace or detrimentally affecting the surface condition of the strip.

While we have shown but two embodiments of our invention, other adaptations and modifications may be made without departing from the scope of the following claims.

We claim:

1. A method for surface-conditioning newly installed continuous silicon steel strip heat-treating-furnace rolls having sleeves of rebonded fused silica thereon during production operation of said furnace which comprises the step of processing production silicon steel strip through said furnace in a relativelydry atmosphere having a maximum dew point of approximately 30F at a temperature in the range of substantially l,400 to 2,000F for a period of approximately 24 to 72 hours with the weight of said silicon steel strip applying a sustained force normal to the peripheral surfaces of said rolls whereby a polished surface free of pores is produced on said furnace rolls.

2. A method as defined by claim 1 in which said temperature is substantially 1,500F.

3. A method for removing roll-pickup fron continuous silicon steel strip heat-treating-furnace rolls having sleeves of rebonded fused silica thereon during production operation of said furnace which comprises the step of processing rough-surfaced silicon steel strip through said furnace in a relatively dry atmosphere having a maximum dew point of approximately 30F at a temperature in the range of substantially 1,400" to 2,000F for a period of approximately 16 hours with the weight of said silicon steel strip applying a sustained force normal to the peripheral surfaces of said rolls whereby a polished surface free of roll-pickup is produced on said furnace rolls.

4. A method as defined by claim 3 in which said temperature is substantially 1,500F.

7. A method as defined by claim 3 in which said silicon steel strip hasa roughened surface finish. 

2. A method as defined by claim 1 in which said temperature is substantially 1,500*F.
 3. A method for removing roll-pickup fron continuous silicon steel strip heat-treating-furnace rolls having sleeves of rebonded fused silica thereon during production operation of said furnace which comprises the step of processing rough-surfaced silicon steel strip through said furnace in a relatively dry atmosphere having a maximum dew point of approximately 30*F at a temperature in the range of substantially 1,400* to 2,000*F for a period of approximately 16 hours with the weight of said silicon steel strip applying a sustained force normal to the peripheral surfaces of said rolls whereby a polished surface free of roll-pickup is produced on said furnace rolls.
 4. A method as defined by claim 3 in which said temperature is substantially 1,500*F.
 5. A method as defined by claim 3 in which said silicon steel strip has a silicon content above 3 percent, by weight.
 6. A method as defined by claim 3 in which said silicon steel strip has an abrasive surface coating thereon.
 7. A method as defined by claim 3 in which said silicon steel strip has a roughened surface finish. 